1.Field of the Invention
This invention relates to a process converting pulp and paper mill waste solids into a papermaking fibrous pulp of substantially the same quality as used by the pulp and paper mill to make its paper products.
2.State of the Art
The manufacture of paper involves the blending of pulp (mainly wood fiber) with clay filler, retention aids such as alum, and other chemicals as deemed desirable, and running the slurry on a paper machine to form a sheet that is dewatered and dried. In this process, fiber and filler escape. Many mills empoly "saveall" devices including polydisc filters, gravity drum filters, vacuum filters, dissolved air flotation cells, and sedimentation basins that capture to some degree the fiber and filler. These solids are then directly recycled.
Despite the presence of "saveall" devices in the paper mill, discharge of fiber and clay from the paper machine area is unavoidable due to leaks, mistakes in operation, or dumping of batches to make grade or color changes on a paper machine. These solids are discharged as a slurry to the sewerage system of the paper mill. The paper mill sewerage system can either discharge to the sewerage system of a municipality or terminate at the paper mill's own wastewater treatment facilities.
In a paper mill wastewater treatment facility, the wastewater would first enter a sedimentation basin or clarifier (called the primary sedimentation basin or clarifier) or in some instances a dissolved air flotation device that captures suspended material (substantially fiber and clay contaminated with random debris) with or without the aid of coagulants or flocculants. The suspended material then forms a concentrated slurry containing normally 1% to 5% solids. This slurry or "sludge" is then pumped to a gravity thickener (if required to further concentrate the suspension) and then to a dewatering device (most normally a belt filter press, vacuum filter, or screw press) that increases sludge solids content normally in the range of 20% to 50%.
Many wastewater treatment facilities also have an aerobic biological treatment process such as the activated sludge process that generates a biological sludge upon further treatment of the wastewater to remove dissolved biodegradable organic materials called BOD.
Mills that make both pulp from wood and paper from the pulp produced (called integrated facilities) will have wastewater containing waste solids from the pulp mill. The pulp mill wastewater may or may not be segregated from the paper mill wastewater. If not, pulp mill solids will be present in the sludge from the primary clarifier.
Therefore differentiation is made between paper mill sludge solids, pulp mill sludge solids, and biological treatment sludge solids. The invention herein described is most useful in relationship to paper mill sludge solids but will also apply to pulp mill sludge solids that contain appreciable quantities of wood fiber. It is advantageous to not mix biological treatment plant sludge solids and non-fibrous pulp mill sludge solids with the pulp and paper mill wastewater fibrous solids. If paper mill, pulp mill, and biological solids are mixed together, the process described by this invention will be successfully applied but the expense and difficulty of processing will be greater than if biological sludge solids and non-fibrous pulp solids were not incorporated with the pulp and paper mill fibrous sludge solids.
The main components of paper mill waste solids are wood fiber and clay contaminated with amounts of random debris. The type of fiber contained in the waste solids is substantially the same as the fiber contained in the pulp used by the mill to make its paper products. However, the waste solids fiber will be inferior compared to the papermaking pulp fiber with respect to fiber length, strength and other physical properties. This fact, along with a high percentage of clay and random debris contained in the waste solids in comparison to the papermaking fiber, makes the waste solids not suitable to be directly recycled. Furthermore, the fiber may contain dyes that make it unsuitable for making certain grades of paper.
Specifically, the sludge fiber contains more fiber fines and clay than the pulp used to make paper. Nevertheless the sludge will contain a substantial portion of long fiber that will be of a quality substantially the same as the fiber used by the mill to make paper provided it can be separated from the fiber fines, clay and other debris contained in the sludge.
In this regard it should be mentioned that fiber emanating from a mill that uses primarily virgin pulps to make paper will likely have a higher % of long fiber in its sludge solids compared to a mill that uses waste paper as its primary source of raw pulp. Waste paper also contains appreciable amounts of clay filler or coating pigments that invariably end up in the wastewater emanating from a paper mill.
Paper mills usually dispose of their sludge in a landfill where they must pay the cost of transportation of the sludge to the landfill and are charged a disposal or tipping fee. Some paper mills dispose of the sludge by landspreading where they must pay the cost of transportation to the landspread site, pay a user fee or purchase land, and spread the material.
A few pulp and paper mills incinerate sludge in dedicated facilities or include it in fuel for existing power boilers. However, the capital cost for dedicated facilities are high and the fuel value of many sludges is low due to high ash and water contents and low heating value of the fiber itself. The steam generated by power boilers has a low intrinsic value.
Others have suggested that the sludge be converted into useful by-products. Conversion of paper mill sludges into valuable by-products is taught, for example, by U.S. Pat. No. 4,481,072 where paper mill sludge is treated with alkali then dewatered. The filtrate is used to prepare white liquor in the Kraft pulping process and the solids added to furnishes used in the manufacture of paperboard and fiberboard.
U.S. Pat. No. 3,876,497 teaches a process of wet oxidation of a sludge to generate energy and recover a clay product useful as a paper making filler.
U.S. Pat. No. 4,356,060 teaches a method of heating the sludge to evaporate water and convert to granules. The granules are then milled and forced through a screen. The product can be used as insulating material or filler in pipe insulation, roofing material, or similar products.
U.S. Pat. No. 4,221,751 teaches a method of molding an article such as a pallet from paper mill sludge.
U.S. Pat. No. 4,285,842 teaches a method for producing a fibrous fire protection agent from sludge.
U.S. Pat. Nos. 3,907,728 and 3,909,470 teach a method of making plywood adhesives extended with paper mill fines.
The invention described differs in that it it teaches a method of separation of high quality fiber from paper mill sludge and the cleaning and bleaching of that fiber such that it can be directly recycled. Therefore, though all the above inventions relate to conversion of paper mill sludge to useful by-products, none are directed at direct conversion of sludge solids into a pulp that can be directly recycled or alternatively used by another paper mill that uses a similar quality pulp to make its paper products.